The present invention relates to an apparatus for determining corrosion activity in a liquid and vapor/condensate corrosion environment. In particular, the corrosive environment is the interior of an oil refinery apparatus such as a pipestill.
Two leading causes of crude oil corrosion on metal surfaces in refinery pipestills are reactive sulfur compounds and naphthenic acids exposed to pipestill (400–800° F.) temperatures. Corrosion mitigation is practiced by dilution with non-corrosive crudes, installation of corrosion-resistant metallurgy, or addition of chemical corrosion inhibitors, either alone or in combination. Reactive sulfur corrosion e.g. from hydrogen sulfide, is recognized as occurring in either the liquid or vapor phase. On the other hand, naphthenic acid corrosion occurs in liquid and in condensate phases, is enhanced in high velocity regions, but is not believed to be a vapor phase phenomenon.
Commercial inhibitors are typically high boiling and, therefore, concentrate in the bulk liquid phase to minimize the risk of distillate contamination. However, multiple injection ports are frequently necessary to ensure that condensation zones in pipestills are contacted by these agents. This adds to the complexity and cost of inhibitor treatment while increasing the risk of contamination to the petroleum distillate streams. The ability to select a chemical agent that can provide metal protection in multi-phase regions with a single injection would make for a more effective treatment while overcoming the cost and potential product quality issues resulting from inhibitor applications at multiple locations.
Efforts to predict pipestill corrosion have primarily been based on corrosion test results, typically carried out in autoclaves. This can provide a measure of metal coupon corrosion in contact with a bulk liquid phase. However, a coupon placed above the liquid is in a fairly static vapor-liquid environment under pressure from enclosed autoclave. This is totally unlike the dynamic vapor-liquid conditions in pipestills, especially under vacuum in vacuum pipestills. Thus, autoclave corrosion tests are not reliable indicators of condensate phase corrosion and therefore inhibitor performance in that phase cannot be properly evaluated.
As a consequence, refineries using corrosion inhibitors monitor the impact on pipestills by insertion of invariably unreliable corrosion probes and by frequent inspections for evidence of corrosion. When the latter is suspected and/or detected, the refiner typically has three options. One is to add more corrosion inhibitor injection points at indicated locations; another is to alloy up those regions, and the third is to lower the crude feed total acid number (TAN) levels. TAN is typically measured by ASTM method D-664 and is expressed in units of milligrams KOH/gram of oil. Oils with TAN values below 0.5 are generally regarded as non-corrosive, between 0.5 and 1.0 as moderately corrosive, and corrosive above 1.0.